1. Field of the Invention
The present invention relates to a granular vinyl chloride resin which is low-dusting and excellent in the powder flowability and which can be used as a paste-making resin, and a process for producing it.
2. Description of the Related Art
For the purpose of automatic weighing or preventing dusting of a paste vinyl chloride resin, it has been attempted in recent years to granulate a fine powder of a paste vinyl chloride resin itself. For example, Japanese Unexamined Patent Publications No. 133409/1990 and No. 133410/1990 disclose a process wherein an aqueous dispersion of a paste-making vinyl chloride resin is dried by means of a spray dryer having a rotary disk atomizer, and the rotational speed of the rotary disk and the temperature and humidity of the drying air during the drying operation, are adjusted to produce a granular resin. However, in this process, if the rotational speed of the rotary disk is reduced to increase the particle size of the granules, drying at a high temperature will be required, and the resulting granular resin tends to be hard and can not readily be dispersed in a plasticizer, whereby a plastisol can hardly be formed.
On the other hand, if the drying temperature is lowered, drying will be inadequate, and a separate drying step will be required again, or a spray drying apparatus of a large size will have to be installed. Thus, an economical disadvantage can not be avoided. In a case where a paste-making vinyl chloride resin is produced by drying an aqueous dispersion containing fine particles only, as has been common in the conventional techniques, a plastisol obtainable from such a resin will have a high viscosity or will be poor in the stability of the viscosity as time passes.
Further, Japanese Examined Patent Publication No. 23180/1973 discloses a process for producing a granular vinyl chloride resin which comprises adding a certain amount of a liquid flocculating agent to a paste-making vinyl chloride resin in a fine powder form, then granulating the mixture by an extruder and drying the resulting granulated product by a fluidized bed dryer. The granular vinyl chloride resin produced by this process is made solely of fine vinyl chloride resin particles and dried at a high temperature at a level of 80.degree. C. and, as such, can not be used for paste-making even if a plasticizer is mixed thereto, and a step of crushing it by a crusher will be required to obtain a plastisol. Even if a plastisol is obtained, its fluidity will not necessarily be satisfactory, and there will be a problem also with respect to the stability with time. Further, a resin granulated by means of an organic liquid or a flocculating agent, is known. However, there is a possible adverse effect of the organic liquid or the flocculating agent to various properties such as durability and printability of the resin itself.
A plastisol prepared from such a resin can hardly be useful for high-speed coating, rotary screen printing, spray coating or dipping, and it has been required to adjust the viscosity, the fluidity, etc. by mixing it with other resins.